The present invention relates generally to an electrophotographic copying apparatus, and more particularly an electrophotographic apparatus of the retention type in which an electrostatic charge image is once formed on a charge retentive member, such as a photoconductive member and an electrically insulating member by means of a well known electrophotography, and a plurality of duplicated copies are formed by repeating successively development and transfer operations without deteriorating or damaging the charge image.
An electrophotographic copying apparatus of such a retention type is known from, for instance U.S. Pat. No. 3,736,055. In addition, FIG. 1 of the present invention is a schematic view showing an embodiment of a copying machine of the retention or multiple-copying type.
In FIG. 1 a reference numeral 1 denotes a photosensitive drum comprising an electrically conductive drum-shaped substrate and a photosensitive layer made of inorganic or organic photosensitive material such as Se, SeTe, CdS, etc. A transparent insulating layer may be applied on the photosensitive layer, if desired. The drum 1 is arranged rotatably in a direction shown by an arrow at a given constant speed. At first, the drum 1 is uniformly charged by a corona charger 2 and then is subjected to an exposure of an image of a document 4 to be copied by means of an optical system 3 to form an electrostatic charge image on the drum 1. The charge image is then developed with a developing agent by a magnetic brush developing device 5 to form a toner image on the drum 1. The toner image is then transported into a transfer section 6 into which a record paper 8 is also fed from a paper cassette 7. The toner image is transferred onto the record paper 8 by a transfer roller 9 made of semi-conductive rubber to which a suitable transfer bias potential is applied. The record paper 8 having the toner image transferred thereto is fed into a fixing device 10 and the toner image is fused onto the record paper 8 to form a final copy. The photosensitive drum 1 is further fed into a cleaning section 11 and residual toner particles on the drum are removed by a cleaning brush 12 and are collected into a suction duct 13. A reference numeral 14 indicates an erasing lamp and a reference numeral 15 a trimming lamp. When it is required to form a plurality of copies from the same and single charge image once formed on the drum 1, the cleaning section 11 is made inoperative by keeping the cleaning brush 12 away from the drum 1 as shown by a dotted line and the drum 1 is subjected repeatedly to the development and transferring operations in succession to form toner images on successively supplied record papers 8.
In order to form a number of copies from the same and single charge image by repeating the developing and transferring operations, it is important that the photosensitive drum 1 has an excellent charge retentive property. That is to say, it is desired that the photosensitive material has a very low dark decay. For instance, the photosensitive layer comprising a Se or SeTe layer and a PVK layer or a thin insulating layer applied on the Se or SeTe layer can be preferably used for the retention type copying machine. Since the photosensitive layer having a high electrostatic capacitance has a very small decay, it is preferable that the thickness of the photosensitive layer is made small. To this end, the thickness of Se or SeTe layer is preferably selected to about 10 to 30.mu..
As explained above, the potential of charge image on the photosensitive layer might be decreased due to the dark decay. Further, it has been found that the charge image might be deteriorated in the magnetic brush developing step due to the fact that conductive or semiconductive carriers contained in the developing agent are made directly in contact with the charge and a leakage current might flow through the carriers.
In order to avoid the above mentioned decay of the charge during the development, it is known to use specially treated carriers. For instance, in Japanese Patent Publication No. 17,198/74, there is disclosed a technique for increasing a resistance of the developer agent by coating the carrier particles with insulating surface layers.
However, when such insulating carriers are used, the developing electrode effect inherent to the magnetic brush development might be decreased and a so-called edge effect might occur to deteriorate a copy image quality. In order to compensate for such an edge effect, it is necessary to take a step such as a prolongation of the developing time, i.e., a decrease in the copying speed, an increase of the number of developments by arranging serially a plurality of magnetic brush developing devices. However, such a measure results in an increase in the cost of the copying apparatus as well as a decrease in copying speed.